The present invention relates to apparatus for transporting webs or strips of paper, textile material, metallic or plastic foil and the like. More particularly, the invention relates to improvements in apparatus of the type wherein the transported web or strip (hereinafter called web) is in contact with the external surface of the cylindrical shell of a roll which can be driven by the web or which is driven by a prime mover.
Web transporting apparatus which are used in printing, stamping and many other machines must advance predetermined lengths of a running web per working cycle of the machine, normally per revolution of the roll which is in contact with and which normally serves to advance the web. Problems arise when one and the same apparatus must transport webs of different thicknesses. In accordance with a presently known proposal, the web which is to be transported by a driven roll is stretched ahead of the region of contact with the roll, and the extent of stretching is a function of the thickness of the web. The stretching of the web takes place within the elastic limits of its material. A drawback of such proposal is that one and the same roll can transport only a limited series of webs having different thicknesses. Furthermore, the extent to which a web of textile material, paper or the like can be stretched within elastic limits is relatively small so that it is not always possible to subject the web to a tensional stress which would be necessary in order to enable one and the same roll to transport relatively thin as well as medium thick and reasonably thick webs.
In accordance with a different proposal, the drive for the transporting roll contains a compensating transmission which can correct the rotational speed of the roll in dependency upon the thickness of the web. A drawback of this proposal is that the compensating transmission is complex, bulky and expensive. Furthermore, the compensating transmission is likely to affect the accuracy of the transporting operation because it tends to impart to the roll periodic vibratory and other stray movements. Such stray movements affect the accuracy of transport of the web and prevent the apparatus from invariably advancing identical lengths of the web per unit of time.